Field of the Invention
The invention relates to a method of regulating inking when printing with a printing machine. When several colors are printed over one another in several printing units, respectively, it has become known to obtain actual values representing the inking with the aid of a detector that is directed towards the printed material at the output of the last printing unit. If photoelectric detectors are employed, the measurement light reflected from specific print control elements, or directly from the printed image, can be keyed in or inputted densitometrically or calorimetrically, converted into an electrical signal and fed to a color control device. From the electrical signal, by applying a mathematical algorithm, respectively, an actual value for the thickness of the printing ink layers present on the printing material can be calculated. Within the color control device, the actual values are compared with desired values. A computer can be used for processing the measured value. If the actual value for the layer thickness deviates from the desired value, a comparison value is used to form a control variable, which is fed to an actuator or adjusting element that effects a change in the layer thickness at the respective measurement location. Conventional printing machines have, for each printing ink, ink-adjusting elements which permit adjustment of a layer thickness in so-called zones transversely to the transport direction of sheets and a web, respectively. The adjustment variables output by the color control device may be varied by an operator or automatically with the aid of ink control pushbuttons assigned to the inking zones. When a change in an adjustment variable is effected by the color control device or manually, a finite time period elapses before subsequent layer thickness changes on the printing material have been completed. The color control device is so constructed that adjustments can be performed correctly only when the layer thickness has essentially reached a steady state value after preceding changes in the adjustment variable. The operator will initiate adjustments manually only when he is convinced that preceding actuating operations have essentially been completed and the printing machine system is in a stable state. In order to avoid unstable regulation with unnecessary adjustments and, if necessary or desirable, to avoid the overshooting of or exceeding the layer thicknesses, it has become known heretofore to purport a selected number of sheets during which adjustments are prevented following a previously initiated adjustment (note the published European Patent Document EP 668 824 B1).
In addition, it has become known heretofore to perform adjustments only when the difference between the actual value and the desired value exceeds a threshold value. The operator has the option of entering or inputting the threshold value, via a keyboard or the like, as a function of the subject and of the desired permissible tolerance of the inking deviations in the printed image.
In order to increase the control speed, it has become known heretofore to determine the gradient of the inking changes in the print and, without waiting for a steady state value, to perform the color control and regulation, respectively, as a function of this gradient (note the published German Patent Document DE 44 12 601 A1). A disadvantage, in this regard, is that a large number of measurements are required in order to determine the gradient sufficiently accurately.
It is accordingly an object of the invention to provide a method of regulating inking when printing with a printing machine wherein a control algorithm is developed which reduces the outlay for obtaining the measured value, avoids erroneous or faulty conditions, improves control accuracy and increases control speed.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method of regulating inking when printing with a printing machine, including determining an actual color value with a color measuring device directed towards a printed material, and feeding the determined actual color value to a color control device, comparing the actual color value with a desired color value, forming an adjusted variable from the comparison value by using a mathematical model of the ink control loop, and feeding the adjusted variable to an ink setting element so that the setting element correctingly changes the inking, which comprises calculating a steady state value (sstab) from an additive superimposition of the time changes in preceding adjusted variable changes (xcex94yi), and calculating a new adjusted variable (y) from the desired color-locus (xdes), the actual color value (xact) and the steady state value (sstab)
In accordance with another mode, the method invention includes continuously adapting the mathematical model for calculating the steady state value (sstab) to current process conditions.
In accordance with a further mode, the method invention includes, at each change to the adjusted variable (y), storing in memory the consecutive number (n) of the print and the magnitude xcex94yi of the change to the adjusted variable (y), the variables (n, xcex94yi) being processed in a mathematical model describing the dependence of the adjusted variables y by a mathematical relationship at the respective time ti of the change to an adjusted variable yi.
In accordance with an added mode, the method invention includes introducing an adjusted variable change (xcex94y) by manual intervention by an operator.
In accordance with an additional mode, the method invention includes determining the actual color value (xact) by having an operator remove a printed copy at a time (ti) predefined by the operator, and measure the actual color value on a measuring device not assigned to the printing machine.
In accordance with a concomitant mode, the method invention includes determining a threshold value for the comparison value (xactxe2x88x92xdes) as a function of the time ti of the determination of the actual color value (xact) and of the desired color value (xdes), and enabling a change in the adjusted variable (xcex94yi) only when the threshold value is exceeded.
The invention offers the advantage that not only the uncertain actual values are used during the calculation of adjusted variables, but steady state values of the variable to be controlled are calculated by using the prehistory of the actuating operations, and are used for control.
A threshold value for the comparison value, from which adjustments are to be performed, is continuously adapted to the current printing conditions, by taking into account the frequency, the duration and the magnitude of preceding actuating operations. The shorter the time interval since the last adjustment, the higher the threshold value is calculated to be. The threshold value can assume an infinite magnitude if the uncertainty of the calculated steady state final values is too high. In this case, no adjustments are permitted. In order to calculate the steady state values and the threshold values, use is made of a mathematical model of the printing machine. It has proven to be advantageous to model a printing machine as a delay element of first order with a dead time. The model of the printing machine is corrected at each measurement of the actual values. The model values are compared with the measured values, and the gain factors of the delay element are recalculated from the corresponding model data and the machine state.
When the method is employed, it is not necessary to register the actual values for the inking continuously, nor to register a large number of actual values for the inking. A single measured data set is sufficient by itself to take into account the steady state final values of the ink layer thickness on the printed material in the respective inking zones. The method can therefore be used in particular in inking machines wherein the measurements of the actual values are performed only sporadically, by sample prints being removed from the regular material flow at any desired times selected by an operator, and being measured. Furthermore, the method makes it possible to take into account the adjusted variable changes performed by the operator without requiring renewed measurements of actual values.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as a method of regulating inking when printing with a printing machine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein: